This invention relates to an ultrasonic ranging system, and to a camera into which such a system is incorporated.
Ultrasonic ranging systems for cameras are disclosed in U.S. Pat. No. 3,522,764, German Patent No. 864,048 and I.B.M. Technical Disclosure Bulletin, Volume 9, No. 7, Dec., 1966, pp. 744-745. In each of the systems, ultrasonic energy is transmitted toward a subject to be photographed, and the subject reflects energy back to the camera. Characteristics of the transmitted and received signals are compared necessitating separate sending and receiving transducers, and a control signal representative of subject distance is produced. The control signal is used to drive the lens mount of the camera to a position functionally related to subject distance whereby the subject will be in focus.
In the U.S. Pat. No. 3,522,764, ultrasonic bursts are transmitted at 40 kHz with a period no less than the time required for sound to travel twice the maximum subject distance for which the lens mount is to be adjusted. Echoes from the subject, with the same periodicity as the transmitted bursts, are received in the intervals between the transmitted bursts, the time between transmission and reception of a burst being related to subject distance and used to establish the duty cycle of a first pulse generator. A second pulse generator, with the same frequency as the first, is associated with the lens mount, but the duty cycle of the second generator depends on the position of the lens mount. Movement of the lens mount takes place until the duty cycles of the two pulse generators are equalized.
In The German patent and in the I.B.M. publication, ultrasonic wave trains are frequency modulated with a period no less than the time required for sound to travel twice the maximum subject distance for which the lens mount is to be adjusted. The echoes from a subject are thus frequency modulated with the same periodicity as the transmitted energy, so that the subject range can be established by the instantaneous difference between the frequencies of the transmitted and received signals.
In addition to requiring two transducers, the known prior art systems have the disadvantages of requiring a considerable amount of time to establish subject range. At room temperature, sound travels at about 340 meters per second so that the time required for a burst to reach a target at 7 meters, which is the maximum distance for which focusing is usually required, is about 20 msec. Thus, each period of the above systems must be about 40 msec; and if ten periods are required to establish subject range, then about 0.5 seconds is consumed in achieving camera focus. Such elapsed time is relatively long with respect to human reflexes, with the result that this photography has to proceed in two successive and distinct steps: one involving focusing, and one involving shutter actuation.
For general ranging purposes, an ultrasonic pulse distance measuring device is disclosed in U.S. Pat. No. 3,454,922 wherein separated fixed frequency bursts are transmitted by a transducer, and a reflected echo is received by the same transducer after a period of time related to target range.
When a fixed frequency ultrasonic sound is utilized, it has been found that subjects within the acceptance angle of the transducer, and within the field of view of the camera, are often undetected. From experimental work, it appears that reflections from various points on the subject may interfere with each other thus cancelling or so weakening the echo at the receiver, that the latter cannot respond, and the subject remains undetected. This phenomenon is most noticeable for subjects that are relatively close to the transducer.
In the context of photographing a subject, the latter is considered to be close to the camera when it is within 2 meters from the camera. Since many photographs are taken with subjects at this relatively close range, the failure of the transponder to receive an echo from a relatively close subject would normally cause the lens mount to improperly positioned.
The side lobes associated with the radiation pattern of ultrasonic transmission give rise to another problem. An off-axis target relatively close to the transducer, and located within an attenuated side lobe of the antenna pattern, may have a surface condition or other characteristic which produces an echo of a strength comparable to a subject located on-axis at a considerable distance from the transducer. In such case, the lens mount of the camera may be set in accordance with the distance to the off-axis target rather than the subject being photographed, and an unfocused exposure would result. While the transducer can be made more directional by increasing its area, this expedient increases the size of the ranging equipment associated with the camera, and neutralizes one of the basic reasons for going to a single transducer whic is to reduce the size and weight of the ranging equipment.
It is therefore an object of the present invention to provide a new and improved ultrasonic ranging system wherein the problems and the deficiencies outlined above are reduced or substantially overcome.
It is a further object of the invention to provide an improved automatic focusing camera.
Another object is to provide a method of ranging for photographic operations.